Flow reversal reactors, in which the flow of the reactants and products is periodically reversed, are well known in the field of chemical engineering. For example, U.S. Pat. No. 4,478,808 to Matros et al. is directed to a method of preparing sulphur trioxide by the oxidation of sulphur dioxide. Matros et al. discloses heat exchange/reaction zones each consisting of a layer of catalyst between two layers of inert heat exchange material. The reaction mixture flow along the catalyst bed is reversed periodically. U.S. Pat. No. 2,946,651 to Houdry is directed to the catalytic treatment of gas streams containing relatively small amounts of oxidizable impurities and discloses a gas permeable bed of solids which operate as a heat exchanger by periodically reversing the direction of flow of gas stream through the bed at such intervals that the hot zone of the bed is maintained generally within the central portion thereof, while the outer portion of the bed with respect to the direction of gas flow is maintained at a relatively low temperature. US Patent Application No. 2009/0101584 to Bos et al. is directed to a reverse-flow reactor comprising at least one catalyst bed which is preceded and followed by at least one bed containing selectively adsorbing material. U.S. Pat. No. 6,019,952 to Haupt is directed to a process for destroying organic contaminants in exhaust gas. Haupt discloses two reactors arranged in parallel where each reactor contains a plurality of serially arranged reaction zones with each reaction zone containing an upstream catalyst, a downstream absorbent and a heater. The arrangement of the catalyst, absorbent and heater may be varied.
In general, flow reversal reactors have heat media zones, the primary function of which is to pre-heat cold reactant gases to the proper temperature before the gases reach the reaction zone (i.e. the catalyst bed). In large-scale industrial reactors operating in dynamic regimes, radial in-homogeneities commonly arise. These radial in-homogeneities result in the formation of radial temperature gradients in the catalytic reaction zones of the reactor in which the central region of the reactor has a temperature greater than the temperature of a peripheral region of the reactor (i.e. near the reactor wall). In some instances, the temperature differential between these two regions may be more than 300° C.
As a result of the temperature differential between the central region and peripheral region of the reactor, two different regimes of operation are found in the reactor. A high temperature regime in the central region of the reactor where chemical conversion approaches 100% and a low temperature regime near the reactor wall where chemical conversion may be less than 30%. The result of this disparity is a decrease in the overall rate of chemical conversion and increased reactor inefficiently.
Several efforts have been made in order to alleviate this problem. For example, Canadian Patent No. 2,192,534 to Ratnani et al., directed to a method and apparatus for performing a gas phase exothermic reaction, discloses a reverse flow reactor in which a combustible feed gas mixture is passed through a first catalyst bed comprising a catalyst material having a low catalytic activity and subsequently passed through a second catalyst bed comprising a catalyst material having a high catalytic activity. However, passing the feed gas mixture through catalyst beds having different catalyst activity sequentially does not necessarily reduce or avoid radial in-homogeneities.
To overcome this issue, it is known in the art to increase the volume of the catalyst bed in industrial reactors. Increasing the volume of the catalyst bed increases the amount of time that the reaction gas is in contact with the catalyst surface, which increases the conversion rate near the reactor wall and therefore increases the total chemical conversion rate of the reactor. However, increasing the volume of the catalyst bed is undesirable economically because it increases the capital costs and operating costs of the reactor due to the need for more catalyst. Additionally, when the concentration of the incoming reactant gas varies, an increase in the volume of catalyst is insufficient to reach a conversion rate of over 95%.
It would therefore be desirable to have a flow reversal reactor in which the effect of radial in-homogeneities is reduced or avoided so that the chemical conversion rate is similar throughout the entire reactor.